There are three types of muscles: cardiac, skeletal, and smooth. Smooth muscles are found in the walls of blood vessels, airways, the gastrointestinal tract, and the genitourinary tract. The caliber of tubes lined by these muscles is dependent on a dynamic balance between the state of contraction and the state of relaxation of the muscles in these organs. Contraction and relaxation of smooth muscles are mediated by different signaling pathways inside the muscles. Pathways which induce relaxation also inhibit contraction. Sustained contraction of muscle is a “spasm” of the muscle. This spasm can be prevented by activating pathways or systems which induce relaxation, or in other words, inhibit contraction.
For the most part, smooth muscles are unique in that they lack the ordered structure of cardiac and skeletal muscles and that they are able to maintain tonic contractions with minimal oxygen use. Pathologic tonic contraction is a state in which the muscles are in spasm.
Many pathological conditions are associated with spasm of vascular smooth muscle (“vasospasm”), the smooth muscle that lines blood vessels. Vasospasm, of the vessel causes narrowing of the vessel lumen, limiting blood flow. Spasm of any vessel leads to ischemia to the organ that the vessel supplies blood to. Ischemia is reversible lack of blood flow and oxygen supply to the tissues. In the case of spasm of the vessels in the heart it leads to cardiac ischemia and/or infarction; spasm of vessels in the brain leads to stroke; spasm of the vessels that supply the intestines leads to mesenteric ischemia, a lack of relaxation of the vessels in the penis leads to impotence, since erection requires vasodilation of the corpra cavernosal (penile) blood vessels; and spasm of the intracranial blood vessels leads to migraines.
Excessive vasoconstriction (or inadequate vasodilation) occur in other disease states as well. Hypertension (high blood pressure) is caused by excessive vasoconstriction, as well as thickening, of the vessel wall, particularly in the smaller vessels of the circulation. This process may affect the lung vessels as well and cause pulmonary (lung) hypertension and asthma (bronchospasm). Other disorders known to be associated with excessive constriction, or inadequate dilation of smooth muscles include toxemia of pregnancy, pre-term labor, pre-eclampsia/eclampsia, Raynaud's disease or phenomenon, anal fissure, achalasia, hemolytic-uremia, and Prinzmetal's angina, a form of coronary spasm that causes angina. Spasm in the coronary arteries also occurs during mechanical manipulation of coronary arteries, such as during angioplasty and stenting. This spasm can lead to ischemia and infarction.
Surgical procedures involving the vasculature are also complicated by vasospasm of smooth muscle, which may result in both short term and long term complications including restenosis and vascular occlusion. There is a general pattern in which vasospasm, if persistent, leads to constrictive remodeling/intimal hyperplasia, and ultimately vascular occlusion. Corrective surgical procedures, such as stenting of a blood vessel, angioplasty, and implanting prosthetic devices such as dialysis access fistulas and shunts, are accompanied by damage to the smooth muscle. This leads to smooth muscle cell proliferation and migration. This ultimately leads to constrictive remodeling and intimal hyperplasia. This process leads to restenosis, prosthetic graft failure, stent and stent graft failure, microvascular graft failure, atherosclerosis, and transplant vasculopathy.
While incompletely understood, intimal hyperplasia is mediated by a sequence of events that include endothelial cell injury and vascular smooth muscle proliferation and migration from the media to the intima. This is associated with a phenotypic modulation of the smooth muscle cells from a contractile to a synthetic phenotype. The “synthetic” smooth muscle cells secrete extracellular matrix proteins, which leads to pathologic vascular occlusion, as described above. Furthermore, increased proliferation and migration of smooth muscle cells can also lead to smooth muscle cell tumors, such as leiomyosarcomas and leiomyomas.
Thus, it would be of great benefit to identify new methods and therapeutics to treat or inhibit smooth muscle vasospasm, promote smooth muscle relaxation, improve other therapies involving smooth muscle, and to treat and inhibit smooth muscle cell proliferation and migration.